The death benefit from a life insurance policy is not taxable to the beneficiary. I doesn't make sense for the insurance company to withhold taxes, unless it was an annuity contract. I suggest you contact the insurance company that withheld the tax to obtain a Form 1099-R, that would show the distribution and the tax withholding. If you can't get that from the insurance company, request a Wage and Income Transcript, which shows data from information returns receive by the IRS such as Forms W-2, 1099, 1098 and Form 5498, IRA Contribution Information. You can get that by going to:
Transcript Types and Ways to Order Them
The only way to resolve the situation is to file a 2019 tax return.
As for your stimulus payments; In order to receive the first two stimulus payments, you will have to file a 2020 tax return and claim the Recovery Rebate Credit. Filing a 2019 or 2020 tax return should indicate eligibility for the third stimulus, if you are eligible.
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